The present invention relates to a process for the aerobic biological treatment of waste water with partial recirculation of the activated sludge, and to a device for implementing the process. In the biological treatment of waste water, the organic contaminants dissolved in the water or present in colloidal form are oxidatively decomposed by microorganisms through their metabolic activity, with oxygen from the air being introduced into the waste water as an oxidation agent and to support cellular respiration. The products of aerobic biological waste water purification are carbon dioxide and water, as well as activated sludge composed of both dead and viable microorganisms and nondecomposed and mineralized waste water components.
It is the general custom to operate aerobic biological waste water purification processes at ambient temperatures. Since the speed of biochemical reactions increases with increasing temperature, many experiments have been conducted to effect waste water purification at temperatures above 45.degree. C. using thermophilic microorganisms. However, high temperature aerobic treatment can only be used in industry in special cases (e.g., stabilization of liquid manure) because thermophilic aerobic microorganisms produce a very finely dispersed activated sludge, which has very poor sedimentation characteristics and is thus very expensive to remove from the waste water. Moreover, the finely dispersed structure of thermophilic activated sludge prevents the formation of a high biomass concentration so that the oxidation output (volume efficiency) of the system is reduced. In addition, the finely dispersed character of thermophilic bacteria does not permit the efficient recirculation of part of the activated sludge, which is the general custom in the waste water purification art, in order to achieve a uniformly high biomass concentration and thus assure optimum decomposition of the waste at certain temperatures.